Sleep regularity

Sleep regularity is the day-to-day consistency of an individual's sleep-wake timing, quantified by the Sleep Regularity Index (SRI) — a score from −100 to 100 (with 0 representing fully random patterns and 100 representing perfectly identical days) that calculates the average probability of being in the same sleep or wake state at any two time points exactly 24 hours apart across a 7-day actigraphy recording. Unlike sleep duration, which measures how long a person sleeps, the SRI captures whether sleep occurs at the same clock times each day, reflecting the stability of circadian entrainment. In a prospective cohort study of 60,977 UK Biobank participants followed for 6.3 years (Windred et al., 2024), participants in the highest SRI quintile had 20–48% lower all-cause mortality risk, 16–39% lower cancer mortality, and 22–57% lower cardiometabolic mortality compared with the least regular quintile; when both SRI and sleep duration were entered simultaneously, duration added no independent predictive value. A separate UK Biobank analysis (Cribb et al., 2023, n = 88,975) confirmed a non-linear relationship, with the lowest SRI percentile (score ≈ 41) carrying a 53% higher all-cause mortality hazard relative to the median. The proposed mechanism is that chronic irregularity disrupts circadian timing — misaligning peripheral organ clocks with the central suprachiasmatic pacemaker — thereby promoting inflammation, hypothalamic-pituitary-adrenal axis dysregulation, autonomic dysfunction, and gut dysbiosis; however, both large studies are observational, so causality remains unconfirmed.